1. Field of the Invention
This invention relates to an optical probe and also to a method of manufacturing the same. More particularly, the present invention relates to a probe to be suitably used in a near field optical microscope for the purpose of emitting evanescent light and also to a method of manufacturing such a probe.
2. Related Background Art
The recent invention of the scanning tunnelling microscope (hereinafter referred to as xe2x80x9cSTMxe2x80x9d) made it possible to visually observe the electronic structure of the surface atoms of an electroconductive specimen [G. Binnig et al. Phys. Rev. Lett., 49, 57 (1982)] so that now a real space image of a specimen is visually observable with an enhanced level of resolution, whether it is crystalline or amorphous. Since then, massive research efforts have been paid on the scanning probe microscope (hereinafter referred to as xe2x80x9cSPMxe2x80x9d) particularly in the technological field of evaluation of fine structures of various materials. The SPM includes the scanning tunneling microscope (STM), the atomic force microscope (AFM) and the magnetic. force microscope (MFM) that are adapted to scrutinize the surface structure of a specimen by utilizing the tunnel current, the atomic force, the magnetic force or the light, whichever appropriate, produced there when the probe thereof having a micro-projection is brought close to the specimen.
Additionally, the scanning near field optical microscope (hereinafter referred to as SNOM) has been developed on the basis of the STM. It detects the evanescent light seeping out of the micro-aperture of the front end of a sharp probe and irradiating the surface of a specimen by means of an optical probe in order to observe the surface of the specimen [Durig et al., J. Appl. Phys. 59, 3318 (1986)].
The photon STM is a type of SNOM also developed recently [Reddick et al., Phys. Rev. B 39, 767 (1989)] and adapted to cause a beam of light to strike the rear surface of a specimen by way of a prism under the condition of total reflection and detect the evanescent light seeping out from the front surface of the specimen by means of an optical probe in order to observe the surface of the specimen.
Probes to be used for near field optical microscopes include those using an optical fiber having a pointed front end and a micro-aperture formed there and those using a cantilever and a probe arranged at the free and thereof for the purpose of irradiation of light or photodetection. Particularly, the cantilever type probe provides various advantages including that a number of probes can be arranged in an array for integration by means of a silicon process and that it can operate as AFM.
An optical probe to be used for a near field optical microscope is provided at the front end thereof with a micro-aperture that has a diameter smaller than the wavelength of light and does not allow any propagated light to pass therethrough. It is adapted to collect optical information with a level of resolution higher than the wavelength of light by means of the evanescent light seeping out from the microaperture. However, the light seeping out from the micro-aperture is very weak and hence requires the use of a high sensitivity detector for detecting scattered light originating from the evanescent light. In other words, for a probe having a micro-aperture, the improvement of the resolution and that of the efficiency and the sensitivity are a sort of trade-off. It is, therefore, highly important to make the light coming from a light source to efficiently get to the micro-aperture of such a probe.
As an attempt for improving the efficiency of exploitation of light, there has been proposed the use of a lens for focussing the light coming from a light source external to the probe to a spot located close to the micro-aperture as the lens is arranged between the light source and the micro-aperture (International Patent Application W09603641A1). The patent document describes a first instance of arranging a probe at the free end of a cantilever and a refractive index lens at a position remote from the cantilever and a second instance of arranging a probe at the free end of a cantilever and a Fresnel lens on the cantilever.
The above cited first instance is accompanied by a problem that the distance between the lens and the micro-aperture varies to shift the focal point of the lens relative to the micro-aperture as the cantilever is displaced because the lens is not located on the cantilever. When observing a specimen by means of a probe, using a cantilever, the gap between the specimen and the probe can be controlled either by holding the is cantilever in contact with the specimen or by oscillating the cantilever while scanning the specimen. However, with either arrangement, the cantilever can be displaced to change the distance between the lens and the micro-aperture. This means that the quantity of light collected by the micro-aperture varies. In other words, the intensity of light irradiating the surface of the specimen fluctuates and the scattered light being observed is affected by the fluctuations to make it difficult to reliably observe the specimen.
On the other hand, the above cited second instance is accompanied by a problem of a poor focussing efficiency due to the use of a Fresnel lens. While a Fresnel lens can be prepared by means of a lithography technique applied to a plane, it Involves a large loss of light due to diffraction of light bending away from the focal point and. scattering of light along the lateral sides of the grating. Additionally, the number of zones that can be arranged on a small cantilever is limited to make it difficult to collect light efficiently. Still additionally, the zone size of a Fresnel lens is highly dependent on the wavelength of light because it is determined as a function of the wavelength of light.
In view of the above identified problems, it is therefore an object of the present invention to provide an optical probe comprising a cantilever, a projection having a micro-aperture and arranged at the free end of the cantilever and a focussing lens also arranged at the free end of the cantilever. With such an arrangement, the distance between the micro-aperture and the lens does not vary if the cantilever is deflected so that the lens shows an improved focussing efficiency. Additionally, the optical probe is practically independent of the wavelength of light.
Another object of the present invention is to provide a method of manufacturing an optical probe, which is simple and provides a good reproducibility and a high processing precision. Optical probes manufactured by a method according to the invention can be arranged in array to form an optical head.
According to a first aspect of the invention, the above objectives are achieved by providing an optical probe including a substrate, an elastic body supported by the substrate and having a free end, a projection having a micro-aperture and arranged at the free end of the elastic body, and a refractive index micro-lens also arranged at the free end of the elastic body and adapted to focus light to the micro-aperture.
According to a second aspect of the invention, there is provided a method of manufacturing an optical probe including the steps of arranging an elastic material on a substrate, forming a refractive index micro-lens in contact with the elastic material on the substrate, forming a junction layer on the elastic material, forming a projection with a micro-aperture on the junction layer, and producing an elastic body having a free end out of the elastic material by removing part of the substrate.